It is known that the tightness of a tank-venting apparatus can be checked during operation of the engine equipped with a lambda controller at lower loads. During operation of the engine at lower loads, a check is made as to whether a lean correction via the lambda controller is necessary because of the vapor supplied to the engine from the tank-venting apparatus. This method is unreliable in that it is possible that the fuel in the tank hardly vaporizes. This has as a consequence that, when regenerating the adsorption filter of the apparatus, the air drawn by suction through the filter is not charged with fuel. For this reason, the lambda controller then makes a correction in the direction of rich in order to add fuel to the air supplied from the tank-venting apparatus.
The California Environmental Authority (CARB) suggested in a directive in 1989 to determine with the aid of a fuel-level sensor and a temperature sensor in the tank whether the motor vehicle on which the apparatus is mounted has been tanked or whether the tanked fuel is so warm that a vaporization of the same can be assumed. When the signals of these measuring sensors in the tank indicate that a flow of fuel vapor into the tank-venting apparatus is to be expected, but then nonetheless no lean correction is determined via the lambda control, then this is to be evaluated as an indication for leakage in the tank. This method has the disadvantage that when the temperature, after which vaporization of the fuel is assumed, is selected to be relatively high, no check as to tightness can be carried out for a longer time span during cold weather and with cold fuel. If the above-mentioned temperature threshold is set lower, then the danger is present that an erroneous statement can be made with respect to tightness when no fuel vapor escapes since fuel has been tanked from which vapor no longer emanates. Based on the measurement data of the sensors in the tanks, a vaporization of the fuel is erroneously assumed; however, such vaporization is not present and, accordingly, no lean correction can be made by the lambda controller.
U.S. patent application Ser. No. 768,973, filed on Oct. 8, 1991, pending, discloses a tank-venting apparatus which includes a pressure difference sensor at the tank and an adsorption filter having a venting line which can be blocked. To check the tightness of the tank, the above-mentioned venting line is blocked and a check is then made with the venting valve open as to whether an underpressure has built up in the tank relative to the ambient pressure. If this is the case, the apparatus is then judged as being tight.
In one embodiment of the method, and when the tank-venting valve is opened, a check is made as to a lean correction by the lambda controller when previously (when the tank-venting valve was closed) an overpressure in the tank was determined.
Notwithstanding the foregoing tank-venting apparatus as well as the methods and arrangements for checking the tightness thereof, the task remains to provide methods and arrangements of the above-described type which are improved as well as tank-venting apparatus which can be easily and reliably checked as to tightness.